Liquid crystal display panels of a transverse electric field mode such the In-phase Switching (IPS) mode or the Fringe Field Switching (FFS) mode have the advantage of being smaller in viewing-angle dependency of gamma-characteristics than liquid crystal display panels of a conventional longitudinal electric field mode (e.g. the VA mode). As such, transverse electric field mode liquid crystal display devices have been widely used as small-to-medium-sized liquid crystal display panels.
Meanwhile, increases in definition of liquid crystal display panels lead to decreases in pixel aperture ratio (ratio of the total area of the pixels to the display region), making it difficult to achieve sufficient display luminance. In particular, small-to-medium-sized liquid crystal display panels for mobile use undesirably become lower in contrast ratio when viewed in lighted environments such as outdoor environments.
Measures have been taken so far by increasing the display luminance through increased backlight luminance and thereby increasing the contrast ratio. However, since increases in backlight luminance undesirably lead to increases in power consumption, these measures taken by raising the backlight luminance are approaching their limits.
A cause of liquid crystal display panels becoming lower in contrast ratio in lighted environments is the reflection of light by the liquid crystal display panels. Given these circumstances, attempts to improve the contrast ratio have been made by suppressing the reflection of light by the liquid crystal display panels.
For example, PTL 1 discloses an IPS mode liquid crystal display panel that includes a phase difference plate (sometimes referred to as “front-side phase difference plate”) provided between a linearly polarizing plate (sometimes referred to as “front-side linearly polarizing plate”) disposed on a viewer's side (sometimes referred to as “front side”) and a liquid crystal cell and thereby prevents light reflected by the liquid crystal cell from being emitted toward the viewer's side. The front-side phase difference plate is configured so that linearly polarized light transmitted through the front-side linearly polarizing plate turns into circularly polarized light that rotates in a first direction and enters the liquid crystal cell. That is, the front-side linearly polarizing plate and the front-side phase difference plate function together as a circularly polarizing plate. When reflected (by the interface where the refractive index changes from being low to being high), the circularly polarized light comes to have its P and S waves both shifted in phase by π radians and, as a result of this, have its direction of rotation reversed. Therefore, light reflected by the liquid crystal cell (transparent substrate) turns into circularly polarized light whose direction of rotation is a second direction opposite to the first direction, and linearly polarized light into which this circularly polarized light is transformed by passing through the front-side phase difference plate is absorbed by the front-side linearly polarizing plate.
The liquid crystal display panel of PTL 1 further includes a phase difference plate (sometimes referred to as “rear-side phase difference plate”) provided between a linearly polarizing plate (sometimes referred to as “rear-side linearly polarizing plate”) disposed on a backlight side (sometimes referred to as “rear side”) and the liquid crystal cell. The rear-side phase difference plate is configured so that when having passed through the rear-side phase difference plate and a liquid crystal layer that is in a black display state, linearly polarized light transmitted through the rear-side linearly polarizing plate turns into circularly polarized light whose direction of rotation is the second direction opposite to the first direction. By passing through the front-side phase difference plate, the circularly polarized light whose direction of rotation is the second direction is transformed into linearly polarized light that is absorbed by the front-side polarizing plate.
PTL 1 provides an IPS mode liquid crystal display panel that can achieve high image quality even when used outdoors.
Meanwhile, as liquid crystal display panels that are suitable for outdoor displays, semitransparent liquid crystal display panels have been known. Each pixel of such a semitransparent liquid crystal display panel includes a reflection mode display region (reflection region) and a transmission mode display region (transmission region). The reflection region is constituted, for example, by using a reflecting electrode as the pixel electrode and making the liquid crystal layer about half as thick as it is in the transmission region. Placing a circularly polarizing plate on the viewer's side makes it possible to perform a reflection mode display with a single polarizing plate.
PTL 2 discloses a liquid crystal display panel characterized in driving at least the transmission region in a transverse electric field mode. The semitransparent liquid crystal display panel described in PTL 2 is configured such that a front-side circularly polarizing plate, a front-side phase difference plate (viewer's-side compensation plate), a semitransparent liquid crystal cell, a rear-side phase difference plate (back-surface-side compensation plate), and a rear-side polarizing plate are arranged in this order. PTL 2 (e.g. paragraphs [0148] to [0158]) describes a liquid crystal display panel including a liquid crystal layer whose initial alignment is in a twisted state. PTL 2 states that the use of a liquid crystal layer whose initial alignment is in a twisted state better suppresses variations in refractive index due to variations in thickness of the liquid crystal layer than the use of a parallel-aligned liquid crystal layer, allowing the front-side phase difference plate to implement satisfactory compensation.